Third Generation Partnership Project (3GPP) Long Term Evolution (LTE) uses Orthogonal Frequency Division Multiplexing (OFDM) in the downlink, where each downlink symbol may be referred to as an OFDM symbol, and Discrete Fourier Transform (DFT)-spread OFDM in the uplink, where each uplink symbol may be referred to as an SC-FDMA symbol. The basic LTE downlink physical resource comprises a time-frequency grid as illustrated in FIG. 1.
FIG. 1 illustrates an example OFDM symbol. The horizontal axis represents time and the other axis represents frequency. Each resource element corresponds to one OFDM subcarrier during one OFDM symbol interval. In the time domain, LTE downlink transmissions may be organized into radio frames.
FIG. 2 illustrates an example radio frame. Each radio frame is 10 ms and consists of ten equally-sized subframes of length Tsubframe=1 ms. For normal cyclic prefix, one subframe consists of 14 OFDM symbols. The duration of each symbol is approximately 71.4 μs.
Users are allocated a specific number of subcarriers for a predetermined amount of time. These are referred to as physical resource blocks (PRBs). PRBs thus have both a time and frequency dimension.
Machine-Type Communication (MTC) is a form of data communication involving one or more entities that do not necessarily need human interaction. MTC is an important and growing revenue stream for wireless network operators. MTC devices, such as monitors, sensors, controls, etc., may also be referred to as MTC user equipment (UE). Operators benefit from serving MTC devices with already deployed radio access technology. For example, LTE is a competitive radio access technology for efficient support of MTC.
Lower cost MTC devices facilitate and expedite implementation of the Internet of Things (IoT). In many applications, MTC devices may require low operational power consumption and may communicate with infrequent and short-duration burst transmissions and small-size data packets. In addition, MTC devices deployed deep inside buildings may require coverage enhancement in comparison to a defined LTE cell coverage footprint.
3GPP defines an MTC UE power saving mode that facilitates longer battery life and an MTC UE category that facilitates reduced modem complexity. Other goals are to further reduce UE cost and provide coverage enhancement. A particular enabler of cost reduction is reducing MTC UE bandwidth to 1.4 MHz in downlink and uplink within any system bandwidth. These MTC UEs may also be referred to as low-complexity UEs or coverage enhanced UEs.
LTE system bandwidth may be up to 20 MHz. The bandwidth is divided into PRBs at 180 kHz. Low-complexity UEs with reduced UE bandwidth of 1.4 MHz are only able to receive a part (up to 6 PRBs) of the total system bandwidth at a time. A group of 6 PRBs may be referred to as a PRB group.
To achieve adequate coverage for low-complexity UEs and other UEs operating delay tolerant MTC applications, time repetition techniques may be used to facilitate energy accumulation of the received signals at the UE. Physical data channels (e.g., Physical Downlink Shared Channel (PDSCH), Physical Uplink Shared Channel (PUSCH)) may use subframe bundling (or TTI bundling). Using subframe bundling, each Hybrid Automatic Repeat Request (HARQ) transmission consists of a bundle comprising multiple subframes, instead of just a single subframe. Repetition over multiple subframes may be applied to physical control channels. The particular number of repetitions may depend on a UE's coverage situation.
A UE may report channel conditions using a Channel State Indicator (CSI). A UE may report CSI aperiodically or periodically.
For aperiodic reporting, a higher layer network element semi-statically configures a UE to feedback Channel Quality Indicator (CQI), Pre-coding Matrix Indicator (PMI), and Rank Indicator (RI) for a PUSCH. The UE may use one of the reporting modes given in Table 1 and described in more detail below.
TABLE 1CQI and PMI Feedback Types for PUSCH CSI reporting ModesPMI Feedback TypeMultipleNo PMISingle PMIPMIPUSCH CQIWidebandMode 1-2Feedback Type(wideband CQI)UE SelectedMode 2-0Mode 2-2(subband CQI)Higher Layer-Mode 3-0Mode 3-1Mode 3-2configured(subband CQI)
Each of the following transmission modes support the corresponding reporting modes on PUSCH:
Transmission mode 1Modes 2-0, 3-0Transmission mode 2Modes 2-0, 3-0Transmission mode 3Modes 2-0, 3-0Transmission mode 4Modes 1-2, 2-2, 3-1, 3-2Transmission mode 5Mode 3-1Transmission mode 6Modes 1-2, 2-2, 3-1, 3-2Transmission mode 7Modes 2-0, 3-0Transmission mode 8Modes 1-2, 2-2, 3-1, 3-2 if the UE is configured with PMI/RIreporting; modes 2-0, 3-0 if the UE is configured withoutPMI/RI reportingTransmission mode 9Modes 1-2, 2-2, 3-1, 3-2 if the UE is configured with PMI/RIreporting and number of CSI-RS ports >1; modes 2-0, 3-0 if theUE is configured without PMI/RI reporting or number of CSI-RS ports = 1Transmission mode 10Modes 1-2, 2-2, 3-1, 3-2 if the UE is configured with PMI/RIreporting and number of CSI-RS ports >1; modes 2-0, 3-0 if theUE is configured without PMI/RI reporting or number of CSI-RS ports = 1
The aperiodic CSI reporting mode is given by the parameter cqi-ReportModeAperiodic which is configured by higher-layer signaling. LTE defines the reporting mode parameter as the enumerated type:
CQI-ReportModeAperiodic ::=ENUMERATED{rm12, rm20, rm22,rm30, rm31,rm32-v1250,spare2, spare1}.
The value rm12 corresponds to Mode 1-2, rm20 corresponds to Mode 2-0, rm22 corresponds to Mode 2-2, etc. PUSCH reporting modes are described in 3GPP TS 36.213. A UE may ignore cqi-ReportModeAperiodic-r10 when transmission mode 10 is configured for the serving cell on this carrier frequency. A UE may ignore cqi-ReportModeAperiodic-r10 configured for the PCell/PSCell when the transmission bandwidth of the PCell/PSCell in downlink is six resource blocks.
For periodic reporting, a higher layer network element may semi-statically configure a UE to periodically feedback CSI components (e.g., CQI, PMI, PTI, and/or RI) on a PUCCH using the one of the reporting modes given in Table 2 and described in more detail below.
TABLE 2CQI and PMI Feedback Types for PUCCH CSI reporting ModesPMI Feedback TypeNo PMISingle PMIPUCCH CQIWidebandMode 1-0Mode 1-1Feedback Type(wideband CQI)UE SelectedMode 2-0Mode 2-1(subband CQI)
Each of the following transmission modes support the corresponding reporting modes on PUCCH:
Transmission mode 1Modes 1-0, 2-0Transmission mode 2Modes 1-0, 2-0Transmission mode 3Modes 1-0, 2-0Transmission mode 4Modes 1-1, 2-1Transmission mode 5Modes 1-1, 2-1Transmission mode 6Modes 1-1, 2-1Transmission mode 7Modes 1-0, 2-0Transmission mode 8Modes 1-1, 2-1 if the UE is configured with PMI/RI reporting;modes 1-0, 2-0 if the UE is configured without PMI/RI reportingTransmission mode 9Modes 1-1, 2-1 if the UE is configured with PMI/RI reportingand number of CSI-RS ports>1; modes 1-0, 2-0 if the UE isconfigured without PMI/RI reporting or number of CSI-RSports = 1Transmission mode 10Modes 1-1, 2-1 if the UE is configured with PMI/RI reportingand number of CSI-RS ports>1; modes 1-0, 2-0 if the UE isconfigured without PMI/RI reporting or number of CSI-RSports = 1
For a UE configured in transmission modes 1-9, a higher-layer signaling network element configures one periodic CSI reporting mode for each serving cell. An example of the higher layer signaling is shown below.
CQI-ReportPeriodic ::= CHOICE { release NULL, setup SEQUENCE {  cqi-PUCCH-ResourceIndexINTEGER (0..1185),  cqi-pmi-ConfigIndexINTEGER (0..1023),  cqi-FormatIndicatorPeriodicCHOICE {    widebandCQI   NULL,    subbandCQI   SEQUENCE {INTEGER (1..4)}  },  ri-ConfigIndexINTEGER (0..1023)   OPTIONAL, -- Need OR  simultaneousAckNackAndCQIBOOLEAN }}
For a UE configured in transmission mode 10, a higher-layer signaling network element may configure multiple periodic CSI reports corresponding to one or more CSI processes per serving cell on PUCCH.
The reporting modes described above, however, may not account for particular attributes of MTC UEs. For example, many MTC UEs only have a single receiving antenna, thus a RI=1 may be assumed.
Additionally, transmission modes may vary for UEs that use Cell Specific Reference Signals (CRS) and those that use Demodulation Reference Signals (DMRS). For example, low-complexity UEs and UEs operating with coverage enhancement support transmission modes 1, 2, and 6 for CRS-based UEs, and transmission mode 9 for DMRS-based UEs.
MTC UEs may also operate using two coverage enhancement (CE) modes. For example, an RRC_CONNECTED UE may operate in either CE mode A or CE mode B. CE mode A specifies a set of behaviors for no repetitions or a small number of repetitions. CE mode B specifies a set of behaviors for a large number of repetitions. The CE mode may be signaled to the UE. For each physical channel, the number or repetitions that can be used in CE mode A or in CE mode B may overlap.
MTC UEs may use frequency hopping. For example, MTC UEs may use frequency hopping for downlink channels such as MPDCCH or PDSCH.
For these reasons, existing CSI reporting modes described above are not adequate to support low complexity, coverage enhanced MTC devices.